Chiral symmetry breaking as open string tachyon condensation

TL;DR

The paper develops a holographic framework for chiral symmetry breaking in confining gauge theories using a brane–antibrane flavor sector with an open string tachyon. The tachyon field in the bifundamental of the flavor group links the UV quark mass and the quark condensate to an IR tachyon profile that drives symmetry breaking, fixes the condensate holographically, and yields anomaly structure, a finite eta′ mass, massless Goldstone bosons for zero quark mass, and linear confinement of highly excited mesons. By incorporating asymptotically AdS UV regions, the model naturally reproduces the Gell-Mann–Oakes–Renner relation for small quark mass and exhibits Coleman–Witten-like behavior in a confining background. The approach offers a dynamical, string-theory-based account of chiral dynamics that matches several qualitative and quantitative features of QCD, while caveats remain regarding the precise tachyon potential and potential background curvature effects, particularly the vector–axial meson slope discrepancy.

Abstract

We consider a general framework to study holographically the dynamics of fundamental quarks in a confining gauge theory. Flavors are introduced by placing a set of (coincident) branes and antibranes on a background dual to a confining color theory. The spectrum contains an open string tachyon and its condensation describes the U(N_f)_L x U(N_f)_R -> U(N_f)_V symmetry breaking. By studying worldvolume gauge transformations of the flavor brane action, we obtain the QCD global anomalies and an IR condition that allows to fix the quark condensate in terms of the quark mass. We find the expected N_f^2 Goldstone bosons (for m_q=0), the Gell-Mann-Oakes-Renner relation (for m_q small) and the η' mass. Remarkably, the linear confinement behavior for the masses of highly excited spin-1 mesons, m_n^2 ~ n is naturally reproduced.